In the interior of an engine built into personal watercraft, oil circulates to lubricate, cool and seal engine components. Lubricating, cooling, and sealing capabilities are varied depending on the temperature of the oil. To achieve sufficient capabilities, it is necessary to properly control the temperature of the oil.
If the engine continues running under a high load, then the temperature of the oil circulating inside thereof rises undesirably excessively. To avoid this, personal watercraft disclosed in Japanese Laid-Open Patent Application Publication No. 2004-360671 includes an oil cooler for cooling oil. A coolant for use in heat exchange is fed to the oil cooler. As the coolant, water outside the watercraft such as sea water and lake water is used. The coolant is also used to cool the engine.
The temperature of the water outside the watercraft is varied depending on season and location. It is difficult to control the temperature of the oil using the coolant which is variable in temperature. Especially, in winter season, the temperature of the water outside the watercraft is often close to zero degrees centigrade. The oil flowing through the oil cooler is cooled excessively by heat exchange with the coolant. In addition, a substantial time lapses until the engine cooled by the coolant is warmed up. Therefore, the temperature of oil circulating inside the engine is not easily increased but a very long time lapses until the temperature of the oil rises to a suitable one and the oil exhibits desired capability.